Does DC bias have an effect on the copper consumption of transformers?

Since the current in the coil and the magnetic flux in the core both contain DC components, they have an impact on both the copper and iron consumption of the transformer.

The main causes of DC bias in transformers are as follows:

1) Geomagnetism resulting from the interaction of the dynamic changes of the solar plasma wind with the geomagnetic field"Storm"。The change of the geomagnetic field will produce a potential gradient on the earth's surface, the magnitude of which depends on the ground conductivity and the severity of the geomagnetic storm, when this low-frequency and long-lasting electric field acts on the neutral grounded power transformer, it will induce geomagnetically induced current in the winding, and when the geomagnetic induced current exceeds a certain limit, it will cause the transformer to be DC biased.

2) DC transmission single large circuit operation, resulting in a large DC current flow underground, so that the surface current will be generated in a certain area around its grounding electrode or converter station, and the transformer in the AC transmission system substation running in parallel with it will be disturbed if it is not far from the converter station, and the direct manifestation of this interference effect is to generate DC component in the excitation current of AC transformer through the grounding neutral point of AC transformer. Cause the transformer to be DC biased.

3. Some scholars believe that the starting current of the transformer along the high-speed rail will cause DC bias to the surrounding grounding transformer; When there is an asymmetrical load in the voltage and current relationship curve of the AC power grid, such as phased AC load, phased rectifier, single-wave rectifier and line commutation inverter, etc., the DC component will also be caused, because some components in the power grid are nonlinear components, resulting in the asymmetry of AC voltage and current waveform, and this asymmetric load will produce DC during operation, and will also cause the DC component to appear in the transformer winding, which will have the same impact on the saturation of the core as when the DC current of the grounding electrode enters the neutral wire of the transformer.

I'll tell you.

This phenomenon can be explained by the familiar saturation magnetization characteristics of the transformer core: the DC current flowing through the windings becomes a part of the transformer excitation current, which makes the transformer core biased, changes the working point of the transformer, and moves part of the original magnetization curve working area to the core magnetic saturation zone, resulting in the total excitation current becoming a spire wave, which ultimately leads to the increase of transformer vibration. Anhui Zhengguang Television Co., Ltd. is committed to solving the problem of DC bias and is a professional service provider in the industry.

Agree with the point upstairs, I'll add one more point.

The loss of the transformer affects the cost of the power grid, the manufacturing cost of the manufacturing plant, and the safety and reliability of the transformer operation, but the parameters of the transformer are mutually restricted. In terms of load loss, in order to reduce it, it is necessary to reduce the current density and increase the wires.

The cross-section reduces the resistance of the winding, but because of the decrease in resistance, the impact coefficient of the short-circuit current increases (because the impact coefficient is inversely proportional to the rk lk value), and the short-circuit current increases. The mechanical force is proportional to the square of the current, due to the increase of the cross-section of the wire, in order to prevent the increase of eddy current in the wire and increase the load loss, it is necessary to use a plurality of small gauge wires in parallel or with transposed wires and combined wires, so that the anti-short-circuit stability of the winding is significantly reduced, so the load loss should not be set too low.

Therefore, the current national standard only lowers the no-load loss repeatedly, from the S9-S15 series of transformers, the no-load is different in various models, and the load loss has not changed at all.

The iron loss of the transformer is the no-load loss.

The copper loss of a transformer is the load loss.

The no-load loss of the transformer is related to the voltage, as long as the transformer carries the rated voltage, the no-load loss is always there.

The load loss is related to the current, that is, the load rate of the transformer, and the load loss is several times greater than the no-load loss when the transformer is fully loaded. However, when the load factor of the transformer decreases, the load loss will also drop significantly, and even less than the no-load loss. In extreme cases, the transformer load is zero, and the load loss is zero.

DC bias is a non-normal working state of a transformerBecause the equivalent impedance of the primary side of the transformer only presents resistance characteristics to the DC component, and the resistance is very small, therefore, the small DC component will form a large DC excitation magnetic potential in the winding, and the DC magnetic potential and the AC magnetic potential act together on the primary side of the transformer, causing the working magnetization curve of the transformer core to shift, that is, the bias phenomenon. The causes of DC bias in transformers are different, mainly including DC transmission and geomagnetic field disturbances

When a DC transmission system is operating at two poles, asymmetrical or unipolar levelsAt this time, the DC current may enter the transformer winding through the neutral point of the transformer, and the DC current will be formed in the transformer winding, and the transformer magnetic dense working point will be offset by the transformer magnetic density working point, so that the transformer will produce DC bias, which will seriously endanger the safe operation of the power system.

Geomagnetic disturbances originate from solar stormsIt is the result of space weather acting on the earth's earth: when the earth's magnetic field changes violently, it will generate an induced geoelectric field in the earth, forming a potential gradient on the surface of the earth, and when the substation grounding point is located at different potentials, a potential difference will be formed between the substations, driving the generation of geomagnetically induced current, and it will also cause the transformer DC bias.

In addition,Subway operation and rail transit can also sometimes cause DC bias in surrounding transformers.

When the HVDC transmission system is running in a single maximum, the DC current flows into the ground through the grounding electrode and flows back to the power supply through the ground. The neutral point of the high-voltage transformer above 110kV is directly grounded, and the resistance of the transmission line is much smaller than the resistance of the earth, so a considerable part of the DC current flows into the transmission line through the neutral point of the transformer; Causes the DC bias problem of the transformer.

At present, there are basically the following solutions:

1. DC current injection method of East China Power Grid, 2. Tsinghua University's series resistance method at the neutral point of the transformer, 3. The capacitor isolation method of the China Electric Power Research Institute at the neutral point of the transformer.

4. Install series compensation equipment on relevant transmission lines.

At present, PAC-50K transformer DC bias device has been installed in Daya Bay Lingao Nuclear Power Station, Taishan Power Plant and other places.

1. DC bias causes the noise of the transformer to increaseWhen there is DC current flowing through the transformer coil, the excitation current will increase significantly. For single-phase transformers, when the DC current reaches the rated excitation current, the noise increases by 10db; If the rated excitation current is 4 times, the noise will increase by 20dB. In addition, the addition of harmonic components to the transformer will cause the noise frequency of the transformer to change, and the noise may increase due to the resonance between a certain frequency and the structural components of the transformer.

DC bias can also cause increased vibration in the transformerThe vibration of the transformer body is mainly caused by the core vibration caused by the magnetostriction of the silicon steel sheet. Magnetostriction causes the core to vibrate periodically with the change of excitation current. The transformer core under DC bias is in a semi-weekly magnetic saturation state, the magnetic flux is offset, and the excitation current is distorted, at this time, the magnetostriction is intensified, resulting in the vibration of the core is also intensified, and there is an electromagnetic attraction caused by magnetic flux leakage at the seam of the silicon steel sheet and between the laminations, and the increase of magnetic flux leakage during magnetic saturation causes the increase of electromagnetic attraction, which also aggravates the vibration of the core.

In addition, DC bias causes the transformer to overheat locallyThe tension plate of the core transformer core or the support plate of the shell transformer core is usually made of magnetic material to obtain sufficient mechanical strength. The core tension plate or support plate located on the surface of the core is the same as the magnetic field strength of the core silicon steel sheet, and its thickness is much thicker than that of the silicon steel sheet, and the large eddy current loss leads to the increase of the temperature of the tension plate (or support plate).

DC bias causes distortion of the transformer's voltage waveformBecause the transformer core may work in the saturation zone when DC bias occurs, the transformer leakage flux increases and the voltage waveform is distorted. In May 2004, Guizhou Gaopo Guangdong Zhaoqing DC transmission in the single large loop operation mode, the 220kV Chuncheng station of Guangdong power grid main transformer neutral point DC current reached, the noise reached, and the total harmonic voltage distortion rate reached.

DC bias causes increased transformer lossesThe losses of transformers include core losses (iron loss) and winding losses (copper loss). Transformer copper consumption includes basic copper consumption and additional copper consumption. Under the action of DC current, the excitation current of the transformer may increase significantly, resulting in a sharp increase in the copper consumption of the transformer.

However, since the main magnetic flux is still a sine wave and the change in magnetic density is relatively small, the impact of DC bias current on the additional copper consumption is relatively small. Transformer iron consumption includes basic iron loss (hysteresis and eddy current loss) and additional iron loss (flux leakage loss). The basic iron consumption is proportional to the square of the magnetic density passing through the core, and is proportional to the frequency.

For transformers with YN,D wiring, the DC current in the transformer windings does not have much effect on the base iron consumption, although the excitation current contains a harmonic component, because the main magnetic flux still maintains the sine wave. However, because the excitation current enters the saturation zone of the magnetization curve, the permeability of the core and the air is close, resulting in a great increase in the magnetic leakage of the transformer. The leakage flux of the transformer passes through the pressure plate, clamps, fuel tanks and other components, and generates eddy current losses in them, i.e., additional iron loss.

The additional iron consumption increases significantly with the increase in the magnetic density of the core.